In the manufacture of ethylene and/or propylene by thermal cracking of a petroleum fraction such as liquified petroleum gas, naphtha or gas oil, substantial quantities of C.sub.4 -hydrocarbon mixtures are produced as coproducts. These C.sub.4 -hydrocarbon streams contain significant percentages of 1,3-butadiene, 1,2-butadiene, butanes and butenes, as well as some highly reactive C.sub.4 -acetylenes (i.e., C.sub.4 -alkynes).
Recovery of 1,3-butadiene and other hydrocarbons in high purity from such C.sub.4 -hydrocarbon streams is well known. However, undesirable impurities such as C.sub.4 -acetylenes require special caution and equipment in recovering the desirable 1,3-butadiene and other hydrocarbons.
These undesirable C.sub.4 -acetylenes are particularly difficult to separate from 1,3-butadiene and other C.sub.4 -hydrocarbons contained in the C.sub.4 -hydrocarbon fraction by fractional distillation or other methods, and must be reduced to a level of parts per million to meet polymerization specifications for 1,3-butadiene or to avoid any subsequent formation of explosive metal acetylides in the process.
A diversity of methods have been developed in the prior art for the selective removal of C.sub.4 -acetylenes from C.sub.4 -hydrocarbon mixtures comprising butanes, butenes and 1,3-butadiene, with the ultimate objective of producing high purity 1,3-butadiene. The prior art methods include selective extraction procedures and selective hydrogenation procedures. In some cases a selective extraction procedure is employed in combination with a subsequent hydrogenation procedure, wherein most of the 1,3-butadiene content is removed before the said hydrogenation step.
U.S. Pat. No. 3,751,508 discloses a novel catalyst which is adapted for selective hydrogenation of acetylenic hydrocarbons in the concurrent presence of diolefinic hydrocarbons. The catalyst consists of a carrier containing at least 5 percent of magnesium-aluminum spinal (MgAl.sub.2 O.sub.4), and an active component consisting of copper and nickel oxides.
U.S. Pat. No. 3,770,619 describes a two stage procedure for selectively hydrogenating an acetylenic type of impurity in a hydrocarbon mixture.
U.S. Pat. No. 3,842,137 describes a process for selective hydrogenation of C.sub.4 -acetylenes in admixture with 1,3-butadiene in the liquid phase which involves reacting the hydrocarbon admixture with a stream of hydrogen diluted to not more than 25 mole percent hydrogen in inert gas, in contact with a Group VIII noble metal supported catalyst.
U.S. Pat. No. 3,859,377 describes the selective hydrogenation of C.sub.4 -acetylenes in admixture with other C.sub.4 -hydrocarbons including 1,3-butadiene, by liquid phase hydrogenation with a hydrogen stream diluted to not more than 50 mole percent in inert gas, in the presence of a palladium on kieselguhr catalyst in which the kieselguhr has a pore volume which is substantially composed of macropores of greater than 700 A.
Other U.S. Pat. Nos. which disclose methods for selective hydrogenation of C.sub.4 -acetylenes in the presence of 1,3-butadiene include 3,897,511; 3,898,298; 3,912,789; 4,101,451; and references cited therein.
U.S. Pat. No. 2,386,927 describes a process for selectively concentrating a conjugated diolefin from other close boiling more saturated hydrocarbons which involves extracting the hydrocarbon mixture with a dialkylamide type solvent to dissolve selectively the conjugated diolefin, and then recovering the diolefin from the solvent phase.
U.S. Pat. Nos. 3,436,436 and 3,436,438 are related references which are concerned with extractive distillation methods for selectively separating a C.sub.4 -diolefin from C.sub.4 -acetylenes and other C.sub.4 -hydrocarbons. Illustrative of one embodiment, U.S. Pat. No. 3,436,438 describes a process which involves subjecting the C.sub.4 -hydrocarbon mixture to extractive distillation with a dialkylamide type of solvent. The C.sub.4 -diolefin is separated as a distillate substantially free of C.sub.4 -acetylenes, and the C.sub.4 -acetylenes are dissolved in the liquid solvent extract phase. The extract phase subsequently is fractionated to separate the C.sub.4 -acetylenes from the solvent. In another embodiment, two extractive distillation stages are employed to yield butanes/butenes, C.sub.4 -diolefin, and C.sub.4 -acetylenes, respectively, as three separate fractions.
U.S. Pat. No. 4,049,742 describes a process for recovering 1,3-butadiene with the aid of a selective solvent from a C.sub.4 -hydrocarbon mixture containing hydrocarbons which are more soluble in said selective solvent than 1,3-butadiene (e.g., acetylenes), and hydrocarbons which are less soluble in said selective solvent than 1,3-butadiene. The C.sub.4 -hydrocarbon mixture is separated by the use of one or more extractive distillation zones, and the fraction containing the acetylenes and other solvent soluble hydrocarbons is subjected to catalytic hydrogenation.
The prior art methods developed for selectively recovering 1,3-butadiene from mixtures containing C.sub.4 -acetylenes and other C.sub.4 -hydrocarbons are generally effective for accomplishing the main objective. However, in terms of large scale plant operation, the prior art methods tend to have high overall capital requirements. They include special equipment and secondary processing procedures which are energy intensive.
It is an object of this invention to provide a process for recovery of high purity 1,3-butadiene from C.sub.4 -hydrocarbon mixtures, wherein the recovered 1,3-butadiene meets the specification requirements of polymerization processes.
It is another object of this invention to provide a process in which the concentration of C.sub.4 -acetylenes in 1,3-butadiene containing C.sub.4 -hydrocarbon mixtures is reduced in a first step, so as to minimize hazards and equipment fouling in subsequent process steps to recover high purity 1,3-butadiene.
Other objects and advantages of the present invention shall become apparent from the accompanying description and example.